The molecular cloning of recombinant MuLV proviral sequences present in thymomas of AKR mice is in progress. DNAs prepared from thymomas induced by the inoculation of 19 AKR/J neonates with MCF-247 viruses were analyzed by Southern blot hybridization. The tumors contain multiple copies of recombinant MuLV provirus DNA sequences in addition to the endogenous MuLV-related sequences present in the AKR/J genome. A DNA library has been constructed from one tumor and a recombinant proviral sequence has been isolated from the library. Molecular subclones from the isolated recombinant containing flanking mouse cellular DNA sequences will be utilized to examine insertion sites in other tumors. Additional libraries from other tumors will also be constructed to evaluate whether common integration regions play a role in the pathology of MuLV induced tumors.